Kallol Mohanta

Magnetic-field-assisted layer-by-layer electrostatic assembly of ferromagnetic nanoparticles

We report that layer-by-layer (LbL) electrostatic assembly of Fe(3)O(4) nanoparticles can be supplemented by orienting magnetic domains of the nanoparticles. With the oriented domains of ionic-capped nanoparticles, both magnetic and electrostatic forces of attraction become operative during the LbL deposition process. The magnetic-field-assisted LbL adsorption process has been evidenced by increased electronic absorbance of the films. While atomic force microscopy studies rule out formation of multiple layers during a single adsorption process, magnetic force microscopy images evidence oriented domains in the LbL films. The results show a novel route for LbL deposition of ferromagnetic nanoparticles with oriented magnetic domains in the thin films.

Diode junctions between two ZnO nanoparticles: current rectification and the role of particle size (and bandgap)

We form junctions between two ZnO nanoparticles of two different dopant concentrations. A monolayer of intrinsic (n-type) and a monolayer of Al-doped (n(+)-type) ZnO nanoparticles are deposited in sequence to form the junctions. The size of the nanoparticles (and hence their bandgap) has been varied. Such junctions on a doped Si electrode have been characterized with a scanning tunneling microscope (STM) tip as the other electrode. The junctions show rectifying current-voltage characteristics. Control experiments, such as (1) symmetric characteristics from the components of the junctions and (2) inverse rectification in a junction having the monolayers in reverse sequence, rule out any effect of interfaces in the observed rectification. The results show that rectification is higher in nanodiodes with high bandgap nanoparticles. The ideality factor of the nanodiodes has been calculated.

Solvent Assisted Tuning of Morphology of a Peptide-Perylenediimide Conjugate: Helical Fibers to Nano-Rings and their Differential Semiconductivity

Understanding the regulatory factors of self-assembly processes is a necessity in order to modulate the nano-structures and their properties. Here, the self-assembly mechanism of a peptide-perylenediimide (P-1) conjugate in mixed solvent systems of THF/water is studied and the semiconducting properties are correlated with the morphology. In THF, right handed helical fibers are formed while in 10% THF-water, the morphology changes to nano-rings along with a switch in the helicity to left-handed orientation. Experimental results combined with DFT calculations reveal the critical role of thermodynamic and kinetic factors to control these differential self-assembly processes. In THF, P-1 forms right handed helical fibers in a kinetically controlled fashion. In case of 10% THF-water, the initial nucleation of the aggregate is controlled kinetically. Due to differential solubility of the molecule in these two solvents, elongation of the nuclei into fibers is restricted after a critical length leading to the formation of nano-rings which is governed by the thermodynamics. The helical fibers show superior semi-conducting property to the nano-rings as confirmed by conducting-AFM and conventional I-V characteristics.

Diode junctions between two ZnO nanoparticles: Mechanism of rectification

We form junctions between two ZnO nanoparticles. Such junctions are formed by electrostatic adsorption of two different monolayers in sequence. While one of the monolayers contains intrinsically n-type ZnO nanoparticles, concentration of Al dopant in the other monolayer is varied. All the junctions show current rectification. In one of the several control experiments, direction of rectification in a junction reverses when sequence of the components in the junction is inversed. This rules out any effect of interface or metal electrode in the observed current rectification. We study the mechanism of rectification that occurs in such a narrow junction. From the current-voltage characteristics, we evaluate the ideality factor of the diodes and find that a recombination current predominates in these junctions in the voltage range of our studies. Capacitance-voltage measurements show that a depletion layer has indeed formed between the two nanoparticles.

Dwindling the resistance value of PEDOT:PSS – coated on fabric yarns

Herein we describe by dip coating method to transform typical fabric yarn to conductive fiber. Different types of yarns have been used to coat from a known conductive polymer, Poly (3,4ethylenedioxythiophene) Poly (styrene sulfonic acid). We have optimized the method to have lesser resistance of the conductive yarns. The minimum resistance achieved has a value of 77 Ω/cm. This value is not high as metals but could be comparable to that of metal oxides or semiconducting materials. However, flexibility of yarns and feeling of fabric combining with the conductivity developed in this process is suitable for wearable electronics and also as gas sensors, electromagnetic shielding.

Light-Harvesting Antenna System for Molecular Electronics

Molecular electronics has been growing rapidly to a point where the ambition is to miniaturize conventional electronic devices down to the single-molecule scale. We construct a molecular scale electronic device based on a donor-acceptor antenna (DAA) system composed of an electron donating quantum dots (QDs) and electron accepting C60 coupled via an aminoalkanethiol bridge. The DAA system was fabricated by a self-assembly procedure, and the charge transfer (CT) rate and charge discharge (CD) efficiency are demonstrated. The CT rate that we obtained for our DAA system is 776 × 106 s-1, which is ten times greater than the reported value. Photoluminescence excitation data, PL lifetime, and intensity trajectory data show that strong CT occurs from the QD to the C60 PTA, and this thin DAA film (<;100 nm) device can hold photogenerated charges for almost 3 h before complete dissipation when not connected to an external circuit, which can have a positive impact on the application of these DAA-based device in molecular electronics. We anticipate that our findings will catalyze the development of new lightweight solar battery.

Solvent Directed Morphogenesis and Electrical Properties of a Peptide–Perylenediimide Conjugate

Molecular organization of electron-deficient aromatic systems like perylenediimides (PDI) is extremely appealing, as they are potential candidates for organic electronics. The performance of these molecules in such applications primarily depends on the self-organization of the molecules. However, any correlation between the morphology of these self-assembled semiconducting molecules and their electrical performances has not yet been formulated. Herein, for the first time, we have made an effort to find such a correlation by studying the self-assembly, morphology, and their conducting properties for a peptide-PDI conjugate. The PDI conjugate formed fiber-like morphology in relatively nonpolar solvents (THF and CHCl3) while in more polar solvents (HFIP, MeOH, ACN, and acetone), spherical morphology could be found. Interestingly, the self-assembly and the morphologies showed a clear dependence on the solvent polarity. In polar solvents, the conjugate aggregates more efficiently than in the nonpolar solvents, and with decrease in solvent polarity, the dimension of the nanostructures increased. However, in all the tested solvents, irrespective of their polarity, the PDI-peptide conjugate adopts a right-handed helicity. To find a correlation between the morphologies with the conducting property, detailed electrical characterization of these nanostructures was carried out. While no significant change could be observed for the dc conductivities of these nanostructures, the ac conductivities show prominent difference at the low-frequency region. A dispersion of conductivity was observed for the nanospheres due to the polarization effect. A critical correlation between the nanostructures and the activation energy was observed as with decrease in radii of curvature of the aggregates the activation energy increases with an exception in the case of MeOH. The observed results suggest that the long-range transport of charge carriers is less favorable when the aggregates are small and closely packed.

Electrochemical study of UV erosion of Au nanorods by silver nanoclusters (NCs) allows the construction of a NC-sensitized photovoltaic cell

It was reported that gold nanorods (Au NRs), synthesized by the conventional seed-mediated method, show unusual photoresponse to UV illumination due to the presence of silver nanoclusters (Ag NCs) which etch away Au NRs to slowly shred them and make them shorter in length. Here we report the electrochemical experiments supporting this atypical behavior. These studies show that a redox reaction is taking place on the surface of Au NRs in the presence of excited Ag NCs. The excitation energy of Ag NCs can be released in the system as the flow of free charges and it can be utilized for a photosensitive electronic application. A NC-sensitized photovoltaic cell shows the possibility of conversion (with 0.17% efficiency) from UV radiation to electrical energy based on Ag NCs.